sfxge(4): use correct name for frame truncation event

The RX_ECC_ERR flag in RX events was misnamed, as it
reported frame truncation. Use the new RX_TRUNC_ERR
name for this flag.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18181